1. Field of the Invention
The present invention relates to a color image-forming apparatus for forming a color image by electrophotography, electrostatic recording, or the like and, more particularly, a color image-forming apparatus capable of forming color images with a high gloss.
2. Description of the Related Art
In the related art, in the color image-forming apparatus of this type for forming color images by electrophotography, electrostatic recording, or the like, if the color image is formed on a base material, for example, if the color copy is taken, the following image forming steps are carried out. That is, light is irradiated onto a color document, then the reflected-light image is read and color-separated by a color scanner, then predetermined image processing and color correction are applied by an image processing device, then the semiconductor laser or the like is modulated based on the resultant image signal of plural colors to emit a laser beam from the semiconductor laser or the like modulated in response to the image signal. A plurality of electrostatic latent images are formed by irradiating the laser beam onto an inorganic photoreceptor made of Se, amorphous silicon, or the like or an organic photoreceptor, which employs phthalocyanine pigment, bis-azo pigment, or the like as the charge generating layer, plural times color by color. A plurality of electrostatic latent images formed on the inorganic or organic photoreceptor are developed sequentially by four-color toners consisting of yellow (Y), magenta (M), cyan (C), and black (K), for example. Then, the formation of the color image on the base material is carried out by transferring the developed toner image from the inorganic or organic photoreceptor to the base material such as paper and then heating/fixing the image by a fixing device such as a thermally-fixing roller, or the like.
The color toner employed in such color image-forming apparatus is fabricated by adhering the fine grains, which have an average diameter of about 5 to 100 nm, onto the particles, which have an average particle diameter of 1 to 15 μm. For example, such fine grains are inorganic fine grains made of silicon oxide, titanium oxide, aluminum oxide, etc. or finely divided resins made of PMMA, PVDF, etc. For example, such particles are prepared by dispersing a coloring agent into a binding resin that are formed of polyester resin, styrene/acryl copolymer, styrene/butadiene copolymer, or the like. Also, for example, the above coloring agents are benzidine yellow, quinoline yellow, Hansa yellow, etc. for the yellow (Y) toner, Rhodamine B, rose Bengal, pigment red, etc. for the magenta (M) toner, phthalocyanine blue, aniline blue, pigment blue, etc. for the cyans (C) toner, and carbon black, aniline black, blend of color pigments for the black (K) toner.
The surface of the color image has a gloss to some extent since such surface of the color image being formed of the above color toner is smoothed in heating/fixing, while normally the surface of the paper does not have a gloss. Thus, the color image has a glossiness that is different from the surface of the paper. Also, it is known that the viscosity of the toner is changed in heating/fixing according to type of a binding resin used in the color toner, the heating/fixing system, etc. and thus the glossiness of the color image is changed.
Meanwhile, the taste in glossiness of the color image is different according to the type of the image, the purpose in use, etc., and a variety of tastes in glossiness are present. In the case of a photographic print such as a portrait, a scenery, etc., normally there is a tendency that an image with a high gloss is preferred from the viewpoint of getting the vivid and crisp image.
Therefore, in the color image-forming apparatus, the technologies disclosed in JP-A-Hei.5-142963, JP-A-Hei.3-2765, JP-A-Sho.63-259575, etc., for example, have already been proposed as the technology for preparing images with a high gloss. The effect such that the image with a high gloss can be obtained by employing a color copying machine and selecting the material of the toner, the fixing conditions, etc. is set forth in these Publications.
However, in the case of the technologies disclosed in these Publications, the glossiness of the image portion made of the toner can be enhanced. Nevertheless, these technologies have problems that the glossiness of the non-image portion cannot be increased higher and also that the glossiness on the base material cannot be made uniform. Also, the unevenness of the color toner remains on the surface of the image, and thus the color image does not become smooth unlike silver halide photographic prints or printed matters. Therefore, these technologies have the problem that a sufficiently smooth appearance cannot be obtained.
Therefore, in order to deal with the above problems, in JP-A-Sho.63-58374, JP-A-Hei.4-278967, JP-A-Hei.4-204670, JP-A-Hei.5-232840, JP-A-Hei.7-72696, etc., for example, the device for transferring/fixing transparent toner as well as color toners onto the base material has been proposed.
However, in the case of the apparatus according to these proposals, since the transparent toner image is formed on the based material, a total amount of the transfer toner consisting of the color toner and the transparent toner is increased and thus the good transfer efficiency cannot be obtained. Therefore, not only the smooth image cannot be obtained, but also the unevenness of the image remains because the color toner is developed. As a result, these technologies have the problem that the smooth image of high surface uniformity cannot be obtained.
Also, as disclosed in JP-A-Hei.11-202583, in order to develop the transparent toner between screen lines comprising color toners, a technique has already been proposed that the image signals for developing the transparent toner are set to be inserted into the clearances between the pixels of the color toner.
However, actually it is difficult to develop the transparent toner in the clearances between the pixels of the color toner due to speed variation of a photoreceptor, speed variation, expansion, and contraction/meandering of an intermediate transfer material and a base material etc. Therefore, this technique has the problem that the image surface structure is not smooth and that, due to poor surface smoothness, images similar to silver halide photographic and lithographic ones, which have a highly uniform appearance cannot be obtained.
In addition, in JP-A-Hei.5-158364, the apparatus that can form the image with high gloss, such as the silver photography by heating/melting the base material, on which the color toner image and the transparent toner image are formed, by the belt-type fixing device and then cooling/releasing the base material is disclosed.
However, in the case of this apparatus, there is the problem that the height difference becomes prominent at the boundary between a high density portion and a low density portion, and especially a small low density spot in the high density portion becomes hollow such that a pit is opened. This phenomenon becomes prominent when speed of a base material passing through a fixing device is enhanced. Thus, this apparatus has the problem that it is impossible to achieve both of the high printing speed and the highly glossy uniform image.
Therefore, the present invention has been made to overcome the problems in the related art. It is an object of the present invention to provide a gloss-imparting device capable of forming an image, whose surface is smooth over an entire area of the image and which has a high gloss independent of the image density, at a high speed not to make height difference at a boundary between a high density portion and a low density portion prominent, and a color image-forming apparatus using the same.